The present invention concerns in general catalysts useful for either catalytic reduction or catalytic oxidation reactions, and more particularly, to electrocatalysts useful as electrodes in fuel cells.
The increasing need for power generation based on non-fossil fuels and with low emissions of pollutants is expected to favor the employment of fuel cells in applications for transportation and power generation.
Fuel cells efficiently convert chemical energy stored in a fuel to electricity through an electrochemical reaction between the fuel and an oxidant (normally oxygen in air), where the reactants are supplied to a pair of electrodes separated by and in contact with an electrolyte, which may be solid or liquid and which transports ions from one of the electrodes to the other, while electrons generated at one electrode are transported to the other electrode through an external load thus producing an electrical current. The oxidation of the fuel takes place on an electrode called the anode, whereas the reduction of the oxidant takes place on an electrode called the cathode. Fuels used in fuel cells may be of different types which may require different operation temperatures and specific designs of the fuel cell to be efficiently converted. Hydrogen, methanol and dimethyl ether are desirable fuels because they can be readily converted at low temperature. Hydrogen is problematic to obtain free from trace amounts of carbon monoxide, which may decrease the conversion efficiency of the fuel on the anode due to poisoning of the catalyst, and in addition hydrogen is problematic to store and transport efficiently.
Methanol and dimethyl ether may be more easily stored and transported than hydrogen, but may also form reaction byproducts, such as carbon monoxide during reaction and in addition may induce lower conversion efficiencies at the cathode if they leak through the electrolyte and there either consume oxygen or poison the cathode catalyst thus rendering it less efficient for oxidant reduction.
For practical purposes the electrocatalysts should preferably be tolerant to poisoning of trace amounts of reaction byproducts or impurities in the fuel or the oxidant stream and to non-desired diffusion of fuel or oxidant across the electrolyte. This means that the catalyst should preferably not react with or catalyse reaction of the compound in question with oxygen but instead remain unaffected by its presence and thus allow for its venting out with the product stream.
The electrodes are typically made up of an electrically conducting electrode substrate and a catalyst layer coated onto the surface of the substrate. The state-of-the-art electrode catalyst typically constitutes finely divided particles of metal, such as platinum or alloys with platinum, with the size of a few nanometers, dispersed on the electrode substrate, typically a carbon powder, to catalyze the desired electrochemical reaction.
The overall fuel conversion rate of an electrode is the combination of the specific activity of its catalytic active sites, the so called turn-over-frequency, and the number of such active sites present in the electrode structure.
In operation of a hydrogen-fuelled fuel cell, hydrogen is provided to the anode electrode where it is oxidized, and protons and electrons are formed. The protons and electrons thus formed are transported through the proton-conducting electrolyte and the external current lead, respectively to the cathode electrode, to which oxygen is provided and reacts with the electrons and protons from the anode to form water. The water thus formed needs to be transported away from the cathode electrode to avoid mass transport limitations of the oxygen to the catalyst on the cathode.
To achieve an operational fuel cell, the structure of the electrodes needs to be designed such that they provide an interface between the three phases (gas, liquid and solid) at which the reactants, electrons and protons meet and react and where the product forms at different stages of the operation of the fuel cell.
Platinum is an expensive metal and a very limited natural resource, which is why alternative electrocatalysts are being sought. Metal-containing macrocyclic compounds, such as, N4-chelate compounds like metalloporphyrins, porphyrins, phtalocyanines and tetraazaannulenes have been found active as electrocatalytic active sites for reduction of oxygen with very high 4-electron transfer properties. See, for example, Bezerra et al., Electrochimica Acta, Vol. 53, pp. 4937-4951, 2008. Combinations of more than one such metal-containing macrocyclic compound have been found to result in cathode electrocatalysts that are fuel tolerant. However, these types of metal-containing macrocyclic compounds have not been shown possible to incorporate efficiently in sufficiently high amounts in electrodes to render the reactant conversion over the catalyst practically useful for their application in electrodes.